Lung infections are responsible for the greatest burden of disease worldwide and the greatest number of infectious disease deaths in the U.S. Regulation of innate immunity is essential - insufficient innate immune responses exacerbate infection, whereas excessive responses cause lung injury. Our long-term goals are to elucidate mechanisms regulating innate immunity in the lungs, to rationally design therapies for improving host defenses and preventing lung injury during infection. Innate immune responses are coordinated by cytokines, and cytokine expression is regulated by NF-?B. Although NF-?B regulates innate immune responses in the lungs, which NF-?B proteins perform which functions in which cells during which infections remain to be determined. Based on our observations of pneumonias in mice deficient in the NF-?B proteins RelA or p50 and on our studies of NF-?B activities in alveolar epithelial cells and macrophages, we propose the central hypothesis that NF-?B in alveolar epithelial cells regulates innate immune responses during bacterial pneumonia and depends upon upstream NF-?B in macrophages during select infections. This will be tested by pursuing the following specific aims: (1) Test the hypothesis that NF-?B RelA in alveolar epithelial cells is necessary for effective innate immunity during both S. pneumonias and E. coli infections. (2) Test the hypothesis that NF-?B p50 in alveolar epithelial cells but not in macrophages limits expression of innate immune cytokines during both S. pneumoniae and E. coli infections. (3) Test the hypothesis that the activation of NF-?B in alveolar epithelial cells requires macrophage-derived TNF and IL-1 during S. pneumoniae but not E. coli infections. (4) Test the hypothesis that NF-?B RelA in macrophages is necessary for effective innate immunity during S. pneumoniae but not E. coli pneumonias. These studies will apply innovative approaches (including cell-specific deletion of RelA in living mice using the Cre/loxP system, assessment of NF-?B activation in vitro using scanning cytometry, assessment of NF-?B activation and promoter activity in vivo using transthoracic electroporation, and p50 knock down using lentiviral vector delivery of shRNA) to determine subunit specificity, cell type specificity, and microbe specificity for NF-?B regulation and function during pneumonia. Results will demonstrate whether RelA in alveolar epithelial cells is essential to host defense, balanced by p50, stimulated by bacteria (E. coli) or rather cytokines (elicited by S. pneumoniae), and dependent on upstream activation of RelA in macrophages during pneumonia resulting from pneumococcus but not Gram-negative bacteria. Such knowledge will be essential to guiding future studies aiming to manipulate innate immune signaling in order to prevent or treat lung infections.